This invention relates to a construction of a transparent sheet material for making transparencies in plain paper electrostatic copiers. More particularly, it relates to a transparency film which utilizes a coating of an electrically conductive polymer to improve acceptance of toner in image areas, thus improving the quality of the transparency.
As is well known, transfer electrostatic copying commonly involves imparting a uniform electrostatic charge, either positive or negative, depending on the specific machine under consideration, to a photoconducting surface which will hold a charge only in the dark, such as a selenium coated drum. This may be accomplished by passing the drum under a series of corona-discharge wires in the dark. The photoconducting surface is then exposed through a lens system to a document or article bearing the image which is to be formed. In areas where light strikes the photoconducting surface the charge is dissipated and flows off through a conducting support to ground, with the electrostatic charge remaining largely intact in the image areas. Next, oppositely charged toner material is brought into contact with the photoconducting surface and clings by electrostatic attraction to the charged areas of the surface. A sheet which is to receive the image is placed over the toner image, and is given a charge, such as by use of corona-discharge wires. As a result, a large portion of the charged toner on the photoconducting surface is transferred to the sheet. Finally, the toner is fused to the sheet by application of heat, pressure, or a combination of both.
Polymeric films have a tendency of acquiring a nonuniform electrostatic charge under certain conditions of contact triboelectric or induction charging. This tendency is undesirable when imaging transparency films in electrostatic copying machines. If charges on such films are not dissipated, toned images become distorted by electrostatic discharges within the copier. In the case of plain paper copiers employing liquid toner, for example, charges on the transparency film cause the liquid to form voids, or bubbles, in the formed images, thus distorting these images. This void-forming phenomenon is known as the "static bubble" effect.
Feeding a stack of plastic film sheets serially into copying machines is difficult because the buildup of electrostatic charges generated as the sheets slide off the stack causes the sheets to adhere to one another. This electrostatic adhesion prevents feeding of the film or causes creep or advancement of the film sheets that are below the uppermost sheet in a stack. Creep can cause jamming or misfeeds. Barker, U.S. Pat. No. 3,618,752 discloses the use of paper adhered to the film sheet as a means for promoting smooth feeding of film sheets. The paper apparently acts to prevent charge buildup, but it increases cost and creates a waste problem. Akman, U.S. Pat. No. 3,854,942 discloses adding a particulate material to a coating to produce a coated surface with raised areas. The use of particulate material separates one film sheet from another, thus reducing the static electrical charge between them.
A receptor film has been made by Minnesota Mining and Manufacturing Company by applying a receptor coating on one side, the image receiving side, of a transparent film base and a coating of antistatic conductive material on the reverse side of the transparent film base. The conductive coating is made from organic ammonium salts in an organic binder. Upon storage in a stack, the conductive coating on one side of one transparent film sheet comes in contact with the receptor coating on the image receiving side of the adjacent transparent film sheet. Under this condition, some of the antistatic conductive material on one transparency film sheet may migrate to the receptor coating of the adjacent transparency film sheet. When the latter transparency film passes through the copier, the areas containing the antistatic material on the receptor surface do not accept toner, thus resulting in speckled images.